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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7667) Support stripe compaction
Date Thu, 20 Jun 2013 06:56:25 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7667?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13688945#comment-13688945
] 

stack commented on HBASE-7667:
------------------------------

Rereading the design doc and how-to-use.  They are very nice.  Can go into the book.

High-level, and I think you have suggested this yourself elsewhere, it'd be coolio if user
didn't have to choose between size and count -- if it'd just figure itself based off incoming
load.

I've seen case where a compaction produces a zero-length file (all deletes) so would that
mess w/ this invariant: "Compaction	must	produce	at	least	one	file	(see	HBASE-6059)." or "...No
stripe	can	ever	be	left	with	0	files..."

I almost asked a few questions you'd already answered above in my previous read of the doc
(smile).

How would region merge work?  We'd just drop all files into L0?  Sounds like we'd have to
drop references if we are not to break snapshotting.

You think this true? "....stripe	scheme	uses	larger	number	of	files	than	
default	to	ensure	all	compactions	are	small,	which	can	affect	very	wide	scans."  Any measure
of how much?

Should stripe be on by default?  Or have it as experimental for now until we get more data?

How to use doc is excellent (though too many configs).  Will review patch again next.


                
> Support stripe compaction
> -------------------------
>
>                 Key: HBASE-7667
>                 URL: https://issues.apache.org/jira/browse/HBASE-7667
>             Project: HBase
>          Issue Type: New Feature
>          Components: Compaction
>            Reporter: Sergey Shelukhin
>            Assignee: Sergey Shelukhin
>         Attachments: stripe-cdf.pdf, Stripe compaction perf evaluation.pdf, Stripe compaction
perf evaluation.pdf, Stripe compaction perf evaluation.pdf, Stripe compactions.pdf, Stripe
compactions.pdf, Stripe compactions.pdf, Stripe compactions.pdf, Using stripe compactions.pdf,
Using stripe compactions.pdf, Using stripe compactions.pdf
>
>
> So I was thinking about having many regions as the way to make compactions more manageable,
and writing the level db doc about how level db range overlap and data mixing breaks seqNum
sorting, and discussing it with Jimmy, Matteo and Ted, and thinking about how to avoid Level
DB I/O multiplication factor.
> And I suggest the following idea, let's call it stripe compactions. It's a mix between
level db ideas and having many small regions.
> It allows us to have a subset of benefits of many regions (wrt reads and compactions)
without many of the drawbacks (managing and current memstore/etc. limitation).
> It also doesn't break seqNum-based file sorting for any one key.
> It works like this.
> The region key space is separated into configurable number of fixed-boundary stripes
(determined the first time we stripe the data, see below).
> All the data from memstores is written to normal files with all keys present (not striped),
similar to L0 in LevelDb, or current files.
> Compaction policy does 3 types of compactions.
> First is L0 compaction, which takes all L0 files and breaks them down by stripe. It may
be optimized by adding more small files from different stripes, but the main logical outcome
is that there are no more L0 files and all data is striped.
> Second is exactly similar to current compaction, but compacting one single stripe. In
future, nothing prevents us from applying compaction rules and compacting part of the stripe
(e.g. similar to current policy with rations and stuff, tiers, whatever), but for the first
cut I'd argue let it "major compact" the entire stripe. Or just have the ratio and no more
complexity.
> Finally, the third addresses the concern of the fixed boundaries causing stripes to be
very unbalanced.
> It's exactly like the 2nd, except it takes 2+ adjacent stripes and writes the results
out with different boundaries.
> There's a tradeoff here - if we always take 2 adjacent stripes, compactions will be smaller
but rebalancing will take ridiculous amount of I/O.
> If we take many stripes we are essentially getting into the epic-major-compaction problem
again. Some heuristics will have to be in place.
> In general, if, before stripes are determined, we initially let L0 grow before determining
the stripes, we will get better boundaries.
> Also, unless unbalancing is really large we don't need to rebalance really.
> Obviously this scheme (as well as level) is not applicable for all scenarios, e.g. if
timestamp is your key it completely falls apart.
> The end result:
> - many small compactions that can be spread out in time.
> - reads still read from a small number of files (one stripe + L0).
> - region splits become marvelously simple (if we could move files between regions, no
references would be needed).
> Main advantage over Level (for HBase) is that default store can still open the files
and get correct results - there are no range overlap shenanigans.
> It also needs no metadata, although we may record some for convenience.
> It also would appear to not cause as much I/O.

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